Method of and control system for controlling a nuclear reactor outlet temperature

ABSTRACT

The invention relates to a control system for controlling an average temperature of a coolant at a reactor core outlet. The control system method detects an average temperature of the coolant at the reactor core outlet, compares the actual average temperature of the coolant at the reactor core outlet with a reference temperature thereby to generate an error signal, and adjusts the actual average temperature of the coolant at the reactor core outlet in response to the error signal. The invention extends to a cascade controller for a nuclear reactor, and to a nuclear power plant.

THIS INVENTION relates to a method of controlling an average temperatureof the coolant at a reactor core outlet. It also relates to a nuclearreactor outlet temperature control system and to a cascade controllerfor a nuclear reactor.

In a nuclear reactor plant, it is desirable that the reactor will notbecome overheated. Accordingly, the rate of neutron generation and theconsequent rate of the fission reaction—the energy of which appears asheat—must be controlled. This is achieved by positioning control rods ofa neutron absorbing material, which are insertable into the nuclearreactor core to a variable depth of insertion.

According to one aspect of the invention there is provided a method ofcontrolling an average temperature of a coolant at a reactor coreoutlet, which method includes the steps of

detecting an actual average temperature of the coolant at the reactorcore outlet;

comparing the actual average temperature of the coolant at the reactorcore outlet with a reference temperature thereby to generate an errorsignal; and

adjusting the actual average temperature of the coolant at the reactorcore outlet in response to the error signal.

Adjusting the actual average temperature of the coolant may includefeeding the error signal, as well as signals corresponding to a fluidicpower and a neutronic power of the reactor to a control rod controlsystem, and controlling the position of the control rods in responsethereto.

The method may include transforming the temperature error signal into apower signal, combining the so-transformed power signal with a measuredreactor neutronic power and a measured reactor fluidic power thereby togenerate a control signal, and adjusting the actual average temperatureof the coolant at the reactor core outlet in accordance with the controlsignal.

Generating the control signal may include transforming a power errorsignal, derived from the power signal, measured reactor neutronic powerand measured reactor fluidic power, into a control rod adjustmentsignal.

Adjusting the actual average temperature of the coolant at the reactorcore outlet may include feeding the control rod adjustment signal into acontrol rod control system and adjusting a control rod insertion depthin response thereto.

According to still another aspect of the invention there is provided anuclear reactor outlet temperature control system which includes

a detector for detecting an actual average temperature of the coolant ata reactor core outlet;

a temperature comparator for comparing the actual average temperature ofthe coolant at the reactor core outlet as detected by the detector witha set point temperature of the coolant at the reactor core outlet andfor generating a temperature error signal;

temperature error signal transforming means for transforming thetemperature error signal into a power signal; and

control rod adjustment means for receiving the power signal and signalscorresponding to a reactor neutronic power and a reactor fluidic powerand adjusting the position of the control rods in response thereto.

The control rod adjustment means may be in the form of a control rodinsertion depth controller for controlling the depth of insertion of thecontrol rods into the nuclear reactor core.

The control rod adjustment means may include a power comparator forcomparing a measured reactor neutronic power, a measured reactor fluidicpower and the power signal, thereby to generate a power error signal.The control rod adjustment means may further include power error signaltransforming means for transforming the power error signal into acontrol rod adjustment signal.

The control system may include reference means, coupled to thecomparator, for providing a manifestation of the set point temperature.

The control system may further include a reactor neutronic power sensorsystem, for sensing the reactor neutronic power and generating a signalof the measured value thereof, and a reactor fluidic power sensorsystem, for sensing the reactor fluidic power and generating a signal ofthe measured value thereof.

By reactor neutronic power is to be understood the rate of neutronformation, and hence the rate of heat generation, in the reactor core.The reactor neutronic power is therefore a variable derived from neutronflux. By reactor fluidic power is to be understood the rate of heattransfer to a working fluid of the reactor. Reactor fluidic power istherefore a function of both the temperature gradient across the reactorcore and the mass flow rate of the working fluid through the reactorcore.

Adjusting the control rod insertion depth results in a correspondingchange in the rate of neutron generation, and therefore in the rate ofthe fission reaction and the reactor neutronic power. The change inneutronic power results in turn in a change in the average temperatureof the coolant at the reactor core outlet.

More particularly, the invention consists of a cascade controller for anuclear reactor, the controller having an inner loop and an outer loop,the inner loop regulating an error between a reactor neutronic power anda reactor fluidic power by manipulating an insertion depth of controlrods of the reactor and the outer loop regulating an average temperatureof coolant at the reactor core outlet by manipulating an error set pointfor the inner loop.

The invention will now be described, by way of example, with referenceto the accompanying diagrammatic drawing, which shows a schematicdiagram of a nuclear reactor outlet temperature control system inaccordance with the invention.

In the drawing, reference numeral 10 refers generally to a nuclearreactor outlet temperature control system in accordance with theinvention.

The control system 10 includes a detector 16 for detecting an actualaverage temperature of the coolant at the reactor core outlet. Thedetector 16 is coupled to a temperature comparator 18. The system 10further includes reference means 17 coupled to the comparator 18, thereference means 17 providing a manifestation of a desired averagetemperature of the coolant at the reactor core outlet, commonly referredto as a set point temperature of the coolant at the reactor core outlet.

In use, the temperature comparator 18 compares an actual averagetemperature of the coolant at the reactor core outlet, as detected bythe detector 16, with a set point temperature of the coolant at thereactor core outlet, as manifested by the reference means 17, andgenerates a temperature error signal in accordance with the comparison.

The control system 10 includes temperature error signal transformingmeans 20 for transforming the temperature error signal generated by thetemperature comparator 18 into a power signal.

The control system 10 further includes a reactor neutronic power sensor22, for sensing a reactor neutronic power, and a reactor fluidic powersensor 24, for sensing a reactor fluidic power. The control system 10also includes a power comparator 26 to which the transforming means 20and each of the sensors 22, 24 are coupled.

In use, the power comparator 26 compares the neutronic power as detectedby sensor 22, the fluidic power as detected by sensor 24 and the powersignal from the transforming means 20, and generates a power errorsignal in accordance with the comparison.

The control system 10 includes power error signal transforming means 40for transforming the power error signal, generated by the comparator 26,into a control rod adjustment signal. The control system 10 includescontrol rod adjustment means 30, in the form of a control rod insertiondepth controller, which is configured to receive the control rodadjustment signal transmitted from the transforming means 40 and toadjust the depth of insertion of control rods of the nuclear reactorinto the reactor core in response thereto.

The control system 10 includes two cascade control loops—an outercontrol loop or temperature control loop, generally indicated byreference numeral 12, and an inner control loop or power control loop,generally indicated by reference numeral 14—that is, an outer controlloop which operates an inner control loop in turn. The detector 16, thereference means 17, the comparator 18 and the transforming means 20 allform part of the outer control loop 12, the reactor neutronic powersensor 22, the reactor fluidic power sensor 24, the comparator 26 andthe transforming means 40 all forming part of the inner control loop 14.An output signal (that is, the power signal) of the outer control loop12 represents a function of the deviation of the actual averagetemperature of the coolant at the reactor core outlet from the set point(or desired) temperature of the coolant at the reactor core outlet. Thispower output signal triggers the inner control loop 14. The innercontrol loop 14 in turn controls the reactor neutronic power, viacontrol rod displacement, in accordance with the output power signal ofthe outer control loop 12.

The input signals for the outer control loop 12 are therefore the actualaverage temperature of the coolant at the reactor core outlet and theset point temperature of the coolant at the reactor core outlet. Anerror of these two input signals is transformed into the power signal,which power signal constitutes the output signal of the outer controlloop 12 and is, in turn, an input signal for the inner control loop 14,together with the measured reactor neutronic power, as sensed by thereactor neutronic power sensor 22, and the measured reactor fluidicpower, as sensed by the reactor fluidic power sensor 24.

In use, the control system 10 is typically activated when the nuclearreactor is in a standby mode or in an operation mode, and duringtransitions between the different operation modes.

The invention extends to a nuclear power plant incorporating a controlsystem in accordance with the invention.

In a nuclear power plant having a reactor unit and a power conversionunit, the reactor unit facilitating the conversion of nuclear energyinto thermal energy which is transferred to the working fluid, and thepower conversion unit facilitating the conversion of thermal energy intoelectricity, the maximum temperature in a closed circuit for the workingfluid, which circuit interconnects the reactor unit and power conversionunit, is set by the average temperature of the coolant at the reactorcore outlet. The control system 10 in accordance with the inventionfacilitates regulation of the maximum temperature in such closedcircuit.

Furthermore, the inventors are aware of the problem of hunting ofreactor nuclear power (and hence of nuclear reactor core outlettemperature) which results in peaks (or spikes) in the nuclear powermagnitude, which peaks may be damaging to the nuclear fuel. Theinventors believe that by making use of the described integratedtemperature controller the problems of hunting and spikes will at leastbe alleviated.

1. A nuclear reactor outlet temperature control system which includes anouter control loop comprising: a detector configured to detect an actualaverage temperature of coolant at a reactor core outlet; a temperaturecomparator configured to compare the actual average temperature of thecoolant at the reactor core outlet as detected by the detector with aset point temperature of the coolant at the reactor core outlet and togenerate a temperature error signal; and temperature error signaltransforming means for transforming the temperature error signal into apower signal; and an inner control loop comprising: a reactor neutronicpower sensor system configured to sense a rate of heat generation withinthe reactor core and generating a signal corresponding thereto; areactor fluidic power sensor system configured to directly sense a rateof heat transfer to a working fluid flowing through the reactor core andto generate a signal corresponding thereto; a power comparatorconfigured to receive and compare the signals from the reactor neutronicpower sensor system and the reactor fluidic power sensor system and toreceive the power signal from the outer loop and compare it with theresult of the comparison between the signals from the reactor neutronicpower sensor system and the reactor fluidic power sensor system andgenerate a power error signal; and control rod adjustment means foradjusting a position of a control rod in response to the power errorsignal.
 2. A control system as claimed in claim 1, in which the controlrod adjustment means is in a form of a control rod insertion depthcontroller configured to control depth of insertion of the control rodinto the nuclear core.
 3. A control system as claimed in claim 1, whichincludes power error signal transforming means configured to receive thepower error signal from the power comparator, transform the power errorsignal into a control rod adjustment signal and feed the control rodadjustment signal to the control rod adjustment means.
 4. A controlsystem as claimed in claim 1, which includes a reference means, coupledto the temperature comparator, for providing a manifestation of the setpoint temperature.
 5. A nuclear power plant which includes a nuclearreactor outlet temperature control system including an outer controlloop comprising: a detector configured to detect an actual averagetemperature of coolant at a reactor core outlet; a temperaturecomparator configured to compare the actual average temperature of thecoolant at the reactor core outlet as detected by the detector with aset point temperature of the coolant at the reactor core outlet and togenerate a temperature error signal; and temperature error signaltransforming means for transforming the temperature error signal into apower signal; and an inner control loop comprising: a reactor neutronicpower sensor system for sensing a rate of heat generation within thereactor core and generating a signal corresponding thereto; a reactorfluidic power sensor system configured to directly sense a rate of heattransfer to a working fluid flowing through the reactor core and togenerate a signal corresponding thereto; a power comparator configuredto receive and compare the signals from the reactor neutronic powersensor system and the reactor fluidic power sensor system and to receivethe power signal from the outer loop and compare it with the result ofthe comparison between the signals from the reactor neutronic powersensor system and the reactor fluidic power sensor system and generate apower error signal; and control rod adjustment means for adjusting aposition of a control rod in response to the power error signal.